Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting devices, arrays, displays, backlights, and segmented displays. Such devices are suitable for many applications, including lighting and sign applications. Organic materials can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The ease of fabricating organic EL devices promises even more efficient and durable materials, which can contribute to further improvements in device architecture.
Organic light emitting devices (OLEDs) function much like inorganic LEDs. Depending on the actual design, light either passes through a transparent electrode deposited on a transparent glass substrate, through a transparent top electrode, or both. The first OLEDs were very simple in that they constituted only a few layers. Recent developments have led to OLEDs having many different layers (known as multilayer devices), each optimized for a specific task.
A performance limitation with some OLED devices is reliability. It has been demonstrated that some of the organic materials are very sensitive to contamination, oxidation, and humidity. Furthermore, some of the metals used as contact electrodes for OLEDs are susceptible to corrosion in air or other oxygen and/or moisture-containing environments.
To obtain efficient OLED devices, low field electron injection generally requires low work function cathode metals, such as Mg, Ca, Li, Ba, or CsF, which readily react with oxygen and water. A low work function calcium cathode, for example, survives only a short time in air due to rapid device degradation caused by atmospheric moisture and oxygen. Such highly reactive metals can also undergo chemical reactions with the nearby organic materials present within the device, which can also have negative effects on the device. Utilizing a low work function material cathode requires careful handling of the device to avoid contamination of the metal, and immediate, high quality encapsulation of the device if operation in the atmosphere is desired.
Many approaches have been attempted in order to solve the problem of device instability and degradation. FIG. 1 illustrates a conventional device. OLED 100 includes substrate 110, anode 120, organic emitting layer 140, cathode 160, and getter 180. Mechanically installed top cover 190 and adhesive sealant 195 are applied to seal the device from the atmosphere. The top cover is often glass, metal, metal film, metallized plastic, or a plastic film with inorganic constituents. The adhesive sealant can be applied in many ways, for example, as a glue or laminate.
In addition to the mechanically applied cover and sealant, a sacrificial or “getter” layer 180 is deposited on the device. Because the getter layer includes a low work function material, such as a metal, that degrades or reacts with any moisture or oxygen that diffuses through the cover/sealant, the getter helps prevent degradation of the lower layers of the device.
However, at least two disadvantages exist for this method. First, pinholes often exist in the getter layer which still provide ample pathways for oxygen and water to reach the electrode metal below. This phenomenon is more completely described in Y. Sato et al., Stability of organic electroluminescent diodes, Molecular Crystals and Liquid Crystals, Vol. 253, 1994, pp. 143–150, for example. When the water passes through the getter and cathode layers, it reaches the emitting layer, causing dark spots in the display. While the combination of mechanically applied covers and adhesives with getter layers slows atmospheric contamination of the device, it may not prevent such contamination.
Second, as the getter layer residing directly on cathode 160 is oxidized and absorbs moisture from the air, it corrodes the cathode. As the cathode is corroded, photon yield or efficiency is reduced, thus darkening the display. Therefore, the lifetime of current organic light emitting devices is limited due to this inability to prevent atmospheric degradation of their highly reactive interior components.
Organic LEDs have great potential to outperform conventional inorganic LEDs in many applications. One important advantage of OLEDs, and devices based thereon, is that they are less expensive than their inorganic LED counterparts. Organic LEDs can be deposited on large, inexpensive glass substrates, or a wide range of other inexpensive transparent, semitransparent or even opaque crystalline or non-crystalline substrates at low temperature, rather than on expensive crystalline substrates of limited areas at comparatively higher growth temperatures (as is the case for inorganic LEDs). The substrates may even be flexible, enabling pliant OLEDs and new types of displays.
As can be seen from the above description, there is an ongoing need for simple and efficient materials and methods for protecting the interior layers of active electronic devices, including organic light emitting devices. The present invention overcomes at least one of the disadvantages associated with conventional devices.